Abstract The microstructure of halite
from the subhorizontal, bedded Main Röt Evaporite Member at
Hengelo, The Netherlands (AKZO well 382, depth interval of 420-460 m),
was studied by transmitted and reflected light microscopy of
gamma-irradiation decorated samples. Primary microstructures compare
favourably with those found in recent ephemeral salt pans. Large,
blocky, fluid-inclusion-poor halite grains and elongated chevrons are
interpreted to have formed in the saline lake stage, while void-filling
clear halite is interpreted to have formed during the desiccation stage
of the salt pan. In addition, in all layers the grains are rich in
deformation-related substructures such as slip bands and subgrains
indicating strains of a few percent. The study of gamma-irradiation
decorated thin sections shows that the main recrystallization mechanism
is grain boundary migration. Grain boundary migration removes primary
fluid inclusions and produces clear, strain-free new grains.
Differential stresses as determined by subgrain size piezometry were
0.45 – 0.97 MPa. The deformation of the salt layers is probably related
to Cretaceous inversion in the area.

1.
IntroductionThe microstructure of natural
rock salt is a product of a complex series of depositional, diagenetic
and deformation processes. Depending on the relative importance of
these, a wide range of microstructures can be formed, starting from the
typical primary structures in young, shallow halite to the strongly
deformed and completely recrystallized tectonites in salt diapirs.
Although both synsedimentary and deformation-related microstructural
processes are documented (e.g. Shearman, 1970; Lowenstein and Hardie,
1985; Urai et al., 1987; Casas and Lowenstein, 1989), details are not
well understood, making the interpretation of halite microstructures
difficult.

One commonly observed
microstructure consists of composite halite grains locally rich in
fluid inclusions, but also containing irregular patches of clear halite
free of fluid inclusions. Not uncommonly, the transition from
fluid-inclusion-rich material to clear halite is defined by a sharp
curved surface within single halite grains. One possible explanation
for this structure is a syndepositional solution-reprecipitation
process in ephemeral or shallow brine system (shallow enough to become
undersaturated with respect to halite when diluted by floodwater). In
an ephemeral salt pan environment, a flooding stage (when floodwater
arrives and partly dissolves the existing halite crust) is followed by
an evaporation stage, in which the irregularly dissolved halite
framework is covered by a new layer of fluid-inclusion-rich grains.
Finally, when the saline lake shrinks and dries out, clear halite
slowly crystallizes in the dissolution voids from the residual
groundwater brine (Lowenstein and Hardie, 1985). After burial, if the halite
is deformed and recrystallized, a similar patchy core-and-mantle
structure can form by grain boundary migration, where the migrating
grain boundaries erase primary fluid inclusion bands. Thus the presence
of patches of clear halite in fluid inclusion-rich grains can also be
explained by a deformation-induced recrystallization process (Urai et
al., 1987).

This problem, i.e.
distinguishing between primary and secondary features in halite, has
been recognized by several authors (Wardlaw and Schwerdtner, 1966;
Hardie et al., 1983; Roedder, 1984) but these authors did not provide
criteria for distinguishing between microstructures resulting from
deformation and those resulting from recrystallization. The aim of this paper is to
present a detailed microstructural analysis of bedded salt from a core
taken near Hengelo, The Netherlands (Fig. 1), in an attempt to separate
primary and deformation-related microstructures, and to quantify the
paleostress in the salt sequence using subgrain size piezometry.

2
Lithostratigraphy of the Röt strata and location of the
study materialSamples described in this
paper are from the Röt Formation in the Twenthe-Rijn concession
area, where more than 400 wells have been drilled for solution mining
since 1919. The Röt Formation (Early Anisian) comprises the strata
between the Solling Formation and the Muschelkalk Formation (Geluk and
Röhling, 1997) (Figs. 2 and 3). The Röt Formation is divided
into two members. At the base lies the Main Röt Evaporite Member,
which consists of four salt layers denoted by the Akzo Nobel company as
salt A to D from bottom to top (Harsveldt, 1980; van Lange, 1994;
Kovalevych et al., 2002). Salt layers A and C are the thickest (25 to
30 m), while layers B and D are only a few meters thick. The D salt
layer is laterally discontinuous, developed as lenses in former
topographic depressions. The salt layers are separated by 1 to 2 m
thick shaly anhydritic mudstones and dolomitic claystones (Harsveldt,
1980; van Lange, 1994) (Fig. 3). Lateral variations in thickness of the
Röt salt are interpreted as syndepositional, reflecting the relief
of the underlying formation, and not due to salt tectonics (Harsveldt,
1980; van Lange, 1994). The depositional environment of the Main
Röt Evaporite Member is interpreted as an enclosed sea with
alternating periods of non-clastic (pure evaporite) deposition and
increased sediment influx (RGD, 1993; van Lange, 1994; NITG, 1998).
Above the Main Röt Evaporite Member lies the Upper Röt
Claystone Member, which is made up mainly of silty claystone with
gypsum and anhydrite nodules (Fig. 3). The thickness of the Upper
Röt Claystone Member is up to 200 m (van Lange, 1994; NITG, 1998).

Fig. 2. a) A large-scale
profile through the Twenthe-Rijn concession area. Note that
displacement along faults can be traced from the Carboniferous basement
(Limburg Group) up to the base of Tertiary. The approximate location of
profile “b” is indicated by the rectangle. b) Profile through the
Twenthe-Rijn concession area with the location of AKZO well 382
indicated. The profile is based on maps of Geluk and Duin (1997) and
Doornenbal et al. (2002). For the position of the profiles see Fig. 1,
and note that the positions are not identical.

The NW-SE trending Gronau
fault zone, situated approximately 15 km NE of the study area, is the
main structural element in the region (NITG, 1998) (Figs. 1 and 2).
Movement along the fault zone occurred since the Carboniferous and this
zone has been reactivated in tectonic phases of Austurian and Saalian,
Late Permian, Early-, and Late Kimmerian, Sub-Hercynian and Savian age
(c.f. NITG, 1998 p. 112). At the local scale, within the Twenthe-Rijn
concession area, a few NW-SE running faults with a maximum displacement
of 50-100 m were reported (Harsveldt, 1980) (Fig. 2). Displacement
along those faults can be traced up to the base of Tertiary, although
their structure within the salt layers is not clear (Geluk and Duin,
1997; Doornenbal et al., 2002). The presence of these faults is
probably related to Cretaceous inversion in the area (de Jager, 2003).
The Tertiary is characterized by tectonic inactivity (de Jager, 2003).

In this paper we describe
samples from AKZO well 382 in the Twenthe-Rijn concession area (Fig.
3). Samples were taken from the “A” salt layer (457.5 m), from the “C”
salt layer (443.5 m and 430.9 m) and “D” salt layer (423.3 m). The
diameter of the core was 10 cm; the length was between 10 and 15 cm.
All photographs and illustrations presented in the paper are oriented
with the top of the section towards the top of the page.

3.
Sample
preparation and methods of studyWe cut 2 x 5 x 8
cm slabs
from the core, parallel to the core axis, using a diamond saw with a
small amount of water (this prevents the development of microcracks in
the halite, without causing significant dissolution artefacts perhaps
due to the Joffé-effect, Joffé, 1928). Microstructures
were decorated by gamma-irradiation in the Research Reactor of
Forschungszentrum Jülich, using a technique similar to that
described by Urai et al., 1985. Two sets of irradiations were carried
out. One irradiation was done at a temperature of 35 ºC with a
dose rate between 1 kGy/h and 3 kGy/h to a total dose of about 1.5 MGy.
The other set was done at a temperature of 100 ºC with a dose rate
between 4 kGy/h and 6 kGy/h to a total dose of about 4 MGy. Depending
on the temperature of the gamma-irradiation the slabs became brown (35
ºC) or blue coloured (100 ºC). The colour intensity seen in
the halite samples is heterogeneous, reflecting the heterogeneous
distribution of solid solution impurities and other crystal defects in
the halite grains (Przibram, 1954; van Opbroek and den Hartog, 1985;
Urai et al., 1985; Garcia Celma and Donker, 1996). After irradiation
the slabs were polished dry on grinding paper, etched with pure water
for 2 seconds and quickly dried with a tissue. This etching technique
removes scratches and provides a micro-relief on the surface of the
slabs improving the stability of mounting. The slabs were mounted on
glass plates at room temperature using epoxy (Körapox 439), and
were cut into thick sections of 4 mm using the wet cutting technique.
The sections were then ground down to a thickness of 1 mm with grinding
paper (dry) and finally etched using the method described by Urai et
al., 1987. The thin sections were studied with reflected and
transmitted, plane polarized light microscopy.

4.
Petrography of the halite samplesTwo types of halite occur in
the samples: a milky, fluid-inclusion-rich halite that comprises up to
40 volume percent of the material, and a clear, fluid-inclusion-poor
halite. The two types commonly occur together, with most halite grains
having one or more milky cores surrounded by clear halite. Some of the
grains have no apparent milky core, and consist entirely of clear
halite. The milky core invariably has a banded structure:
fluid-inclusion-rich bands alternate with fluid-inclusion-poor bands
(Fig. 4). The fluid inclusion bands are about 200 to 400 µm thick
and contain cubic (negative crystal form) fluid inclusions. The
alternating fluid inclusion bands define chevrons, cubes and hoppers.

Chevrons were found in the A,
C and D layers (457.5 m, 443.5 m and 423.3 m). The chevron halite
grains are 0.5 to 2 cm long, vertically elongated grains, which are
very often truncated or rimmed by clear halite (Fig. 5). Commonly, the
clear rim comprises 50-70 volume percent of a chevron grain, rarely up
to 90 percent. The transition from the milky, fluid-inclusion-rich core
to the clear halite is defined by a sharp, curved surface (Fig. 5). In
gamma-irradiated samples usually there is no difference in irradiation
colour between the milky and clear halite within one grain (Fig. 6). In
some cases the top of the chevrons is truncated by layers of anhydrite
and polyhalite partings (Fig. 7).

Fig. 5.
Photomicrographs of
halite from salt layer C (443.5 m) show sharp, commonly curved
boundaries between milky, fluid-inclusion-rich and clear halite regions
within a single grain. The black straight lines seen in the right hand
side image are cleavage planes, introduced during sample preparation.
Plane polarized, transmitted light images of unirradiated samples.
Image width is 2.7 mm.

Fig. 7.
Overview image of a
thin section of gamma-irradiation decorated sample (irradiated at 100
ºC) from salt layer C (443.5 m) photographed in transmitted light.
White patches are chevron halite grains. Grain boundaries occur as
dark, nearly black lines, the white polygons within grains are subgrain
boundaries. Three different layers can be distinguished. Layer 1 (at
the bottom) consists of chevron halite grains and is extensively
truncated by dissolution pipes. The boundary between layers 1 and 2 is
marked by anhydrite and polyhalite partings. The layer 2 is much alike
as layer 1, and also characterized by the presence of chevrons and
clear grains. The boundary between the layers 2 and 3 also
characterized by the presence of anhydrite and polyhalite partings. At
the top (layer 3) large, blocky halite grains occur. New, strain-free
grains, or migrated grain boundaries were rarely observed, suggesting
that deformation-induced recrystallization did not alter the primary
structures. Image width is 4.5 cm.

Cubes and hoppers outlined by
fluid inclusion bands were found in the A, C and D layers (457.5 m,
443.5 m and 423.3 m), although these occur less frequently than
chevrons. The size of cube and hopper grains varies between 0.5 and 3
cm, however the larger grains (1.5 to 3 cm) very rarely contain
well-developed fluid inclusion bands (Fig. 8). Such large, blocky halite
grains were found in the C layer (443.5 m and 430.9 m). In some cases,
small (0.5 to 1.5 cm) grains are intercalated in these large, blocky
grains. These hopper-shaped small grains are visible only by the
different irradiation colouring (less intensively coloured, Fig. 8b).

Clear halite grains, rarely
observed, occur as 0.3 to 1 cm crystals, locally as elongated grains
and usually among chevrons, or rarely as smaller (0.1 to 0.6 cm)
equiaxed grains (Fig. 9).Anhydrite and polyhalite
occur at grain boundaries of the halite grains, and commonly within
grains either as inclusions or as small grains arranged into thin,
curved bands. In many cases anhydrite and polyhalite have a spherulitic
appearance with the laths arranged radially around a central core (Fig.
8a).

The majority of both clear
and milky grains contains subgrains, as shown by etching and
gamma-irradiation (decorated subgrain boundaries were observed only in
slabs irradiated at 100 ºC and high total dose). In most grains,
subgrains occur as equiaxed polygons with triple junctions of about
120° and with an average size between 250 and 400 µm (Fig.
9). In some of the large grains (e.g. C sample, 430.9 m) the subgrains
are arranged into wavy, crystallographically controlled bands (Fig.
10). Much less frequently, elongated subgrains are found in grain
boundary regions. Subgrain-free grains and grain boundary regions were
observed in all samples, comprising up to 5 to 10 % of halite volume.
Subgrain-free grains are relatively small (usually <0.5 cm) and are
equiaxed and fluid-inclusion-free (Figs. 9 and 11). Subgrain-free grain
boundary regions are always narrow (<0.3 cm) and less intensively
coloured by 35 ºC irradiation (Figs. 12 and 13). Gamma-irradiation
decorated bands, believed to be slip lamellae, were observed in a
number of cases in both subgrain-rich and subgrain-free grains (Fig.
14). The direction of the decorated glide planes slightly changes
direction at some subgrain boundaries (Fig. 14). Grain boundaries are usually
serrated at the contact of subgrain-rich grains, and commonly smooth at
the contact of subgrain-rich and subgrain-free grains (Fig. 15). Very
rarely bulged grain boundaries between subgrain-rich and subgrain-free
grains were observed (Fig. 12). Arrays of fluid inclusions are common
on grain boundaries (Fig. 16).

Fig. 11.
Photomicrographs
show the microstructures of sample from salt layer A (457.5 m). Plane
polarized transmitted light images of sample gamma-irradiated at 100
ºC.a)
Migration of high-angle
grain boundaries as recorded by elongated subgrains, and strain-free
new material, which grows at the expense of old, heavily substructured
grains. Image width is 11 mm.b)
Strain-free grains grew at
the expense of deformed ones. The size of some of the new grains is
comparable to that of subgrains. Image width is 7 mm.

Fig. 12.
Microstructures of
gamma-irradiated sample from salt layer A (457.5 m). Image width is 2.7
mm.a) Plane
polarized, reflected
light image of the polished and etched surface. The grain boundary
(NW-SE trending line) is interpreted to have migrated to the SW, as the
area to the NW of the boundary is virtually subgrain-free. The dark
spots at the grain boundary and in the highly substructured grain are
fluid inclusions that decrepitated during sample preparation. Note the
bulged shape of the grain boundary.b) Same
area photographed in
plane polarized transmitted light. The area interpreted as swept by
grain boundary migration is less intensively coloured by 35 ºC
gamma-irradiation. Note that the subgrain boundaries were not decorated
by the irradiation. Also note that the area, which was swept by the
migrating grain boundary, is fluid-inclusion-free.

Fig. 13.
Photomicrograph of
sample gamma-irradiated at 35 ºC from salt layer D (423.3 m).
Minor grain boundary migration occurred in the grain in the middle of
the image (see arrows), where the former {100} crystal face (grain
boundary) migrated. The area swept by grain boundary migration is less
colored. Grain boundaries occur as dark lines. Note that subgrain
boundaries were not decorated with Na-precipitates by 35 ºC
irradiation. Note also that the thickness of the thin section is
approximately 2 mm, so grain boundaries not perpendicular to the plane
of image appear thicker. Plane polarized transmitted light image, image
width is 2.4 cm.

Fig. 14.
Photomicrographs
show microstructure of sample from salt layer A (457.5 m)
gamma-irradiated at 100 ºC. The milky, substructured,
fluid-inclusion-rich grain at the bottom part is interpreted as old
grain, which is replaced by a new, clear, strain-free grain (upper left
corner). Note the presence of decorated planes, interpreted to be glide
planes, in both the old and new grains. Also note how the decorated
glide planes change direction at some subgrain boundaries due to the
slight misorientation of the subgrains. Plane polarized transmitted
light image, image width 6 mm.

Fig. 15.
Microstructures of
sample from salt layer A (457.5 m, gamma-irradiated at 100 ºC),
photographed in plane polarized transmitted light. In both images the
grain boundaries occur as dark curves, while the white, milky areas are
remnants of chevron grains.a) The
clear,
fluid-inclusion-free regions could be explained by a recrystallization
mechanism of grain boundary migration. However, the presence of
subgrains both in the chevrons and in the fluid-inclusion-free region
implies that the structure is syndepositional and is a product of
dissolution and precipitation processes. Image width is 16 mm.b)
Similar structure as shown
in image “a”. Milky, fluid-inclusion-rich (upper left corner) and
clear, fluid-inclusion-free halite are separated by a sharp, curved
line. The presence of subgrains in both the milky and clear parts is
strong evidence that the structure was not developed by grain boundary
migration. Note the serrated grain boundary at the right side. Image
width is 1 mm.

Fig. 16.
Photomicrographs
show the microstructures of a sample from salt layer A (457.5 m,
gamma-irradiated at 100 ºC), photographed in plane polarized
transmitted light. In both images the grain boundaries occur as dark
curves, while the white, milky patches are remnants of fluid
inclusion-rich chevron grains.a)
Elongated subgrains
indicative of grain boundary migration. Grain boundary migration (see
arrows) erases the old, milky, fluid-inclusion-rich part of the old,
deformed grains and produces strain-free regions. Since the strain-free
grains are free of fluid inclusions, it seems very likely that the
fluid inclusions in the deformed and consumed grain were transformed
into the grain boundaries during grain boundary migration
recrystallization. Image width is 8 mm.b)
Similar structure as shown
in the “a” image. A grain with relatively little substructure replaces
the primary, fluid-inclusion-rich, highly substructured grain. Here
again, the migrating grain boundary seems collecting the fluid
inclusions at the grain boundary. Fluid inclusions at the grain
boundary occur as dark spots in the image. Image width is 1 mm.

An important observation is
that the boundary between milky, fluid-inclusion-rich and clear,
fluid-inclusion-free halite can be either inside a grain or at a grain
boundary (Figs. 15 and 16). If it is inside a grain the clear,
fluid-inclusion-free part is in some cases subgrain-rich, in some cases
subgrain-free (Figs. 15 and 16). In many cases grain boundaries are in
contact with a milky, fluid-inclusion-rich part of a grain.

5.
Interpretation and discussion5.1
Syndepositional (primary) structuresAll samples contain
structures interpreted as syndepositional in origin. The truncated
chevron grains and the vertically elongated clear halite grains are
features which compare well with those reported by Lowenstein and
Hardie (1985), who described the characteristic features of salt pan
evaporites. Ephemeral salt pans are normally dry, shallow depressions,
filled with layered halite. The halite layers evolve by repeated cycles
of desiccation, flooding, evaporative concentration and re-desiccation.
The flooding stage brings unsaturated floodwater into the dry salt pan,
and converts it into a brackish lake. The unsaturated water partly
dissolves the old salt crust, preferentially at grain boundaries,
producing a karst-like surface. As the floodwater becomes more
saturated with respect to NaCl due to the dissolved salt and continuous
evaporation, crystallization starts at the brine surface, where halite
hopper crystals and plates form, which sink to the bottom. On the
bottom of the brine pool, the sunken crystals and the old, eroded salt
crystals start to grow in crystallographic continuity with seed
crystals by precipitation from the concentrated floodwater. Growth
competition between the overgrowing crystals produces vertically
elongated chevron halite crystals (Nollet et al., 2005), with upward
directed apices. In the desiccation stage, as the saline lake dries
out, void-filling clear halite crystallizes from the residual
groundwater brine. Lowenstein and Hardie (1985) argue that the salt pan
environment is best identified by the presence of dissolution features.
Clear halite truncating chevron halite grains, and vertically elongated
clear halite grains in the studied cores are interpreted as dissolution
structures and regarded as strong evidence for the salt pan environment.

Syndepositional structures
documented in recent ephemeral salt pans have been extensively reported
in ancient salt deposits (Dellwig, 1955; Wardlaw and Schwerdtner, 1966;
Casas and Lowenstein, 1989; Benison and Goldstein, 2001; for overview
see Warren, 1999). Our interpretation of the synsedimentary structures
in the Röt salt layers is consistent with these previous works
(Fig. 17). The lowermost and uppermost
samples (layer A 457.5 m (Fig. 9) and layer D 423.3 m) are rich in
chevrons and primary, dissolution related-features: chevron halite
layers that formed in the salt pan during concentration of the brine
and clear halite that crystallized in former voids during the
desiccation phase. The relative abundance of clear halite in some
samples may imply that the chevron layer underwent repeated episodes of
dissolution and precipitation.In the C layer (443.5 m) the
elongated, truncated chevron grains, dissolution pipes (Fig. 7), and
the presence of large (1.5 to 3 cm), fluid-inclusion-poor, blocky
halite grains above the truncation surface marked by anhydrite partings
are also features which fit well with the salt pan model. Thus the
chevron halite layers are interpreted as forming by competitive growth
of bottom-nucleated crystals in the saline lake phase, with the
void-filling clear halite crystallized during the desiccation phase.
The horizontal truncation surface is interpreted to be a result of
dissolution caused by arrival of unsaturated flood water. The presence
of chevrons in the bottom layers may be explained by the shallow water
environment, since water shallow enough allows fluctuations in NaCl
saturation level and thus fluctuations in growth rate. The rhythmic
alternation in growth rate results in the development of alternating
fluid-inclusion-rich and fluid-inclusion-poor bands corresponding to
high and low growth rate periods (Roedder, 1984; Handford, 1990). The
blocky crystals are interpreted as forming in concentrated, relatively
deep water (deep enough to prevent the development of fluid inclusion
bands). The intercalated small hoppers in the blocky grains are
probably sunken hopper crystals formed at the brine-air interface
(Arthurton, 1973). The second sample studied from the C layer (430.9 m)
that is made up entirely of large, fluid-inclusion-free, blocky
crystals, has the same appearance as that found in the upper part of
the C layer (443.5 m) sample (Fig. 7). Those large blocky halite
crystals are also interpreted to have formed in concentrated,
relatively deep water in the evaporative concentration stage of the
salt pan.

Fig. 17.
Diagram illustrating
the evolution of the Hengelo rock salt. (a) Overgrowth of seed crystals
with different orientation in the evaporative concentration stage of
the salt pan cycle. Favourably-oriented crystals override those which
have unfavourable orientation. Growth layering (black parallel lines)
is a sequence of alternating fluid-inclusion-rich and
fluid-inclusion-poor bands. The growth of the crystals continues until
the saline lake dries completely out (desiccation stage) (b) Formation
of dissolution surface and cavity due to the arrival of undersaturated
water during the flooding stage. (c) Evaporation concentrates the
undersaturated water, and thin crust of gypsum crystals precipitates
from the concentrated water. With further evaporation the system
arrives back to the evaporative concentration stage and the gypsum
layer is overgrown by a new halite layer. Dissolution voids are filled
with clear halite. (d) Layered halite rock evolved by repeating salt
pan cycles. The remaining pore spaces are completely filled with clear
halite at shallow burial depth. (e) As the salt deforms, the
deformation-induced grain boundary migration recrystallization starts
to erase the primary structures. Due to this process, the
identification of synsedimentary features becomes progressively more
difficult as deformation and recrystallization continue (modified after
Shearman, 1970).

Extreme truncation of
chevrons by clear halite, a common feature in many ancient salt pan
evaporites, is best illustrated by the example of the C core (443.5 m)
(Fig. 7). In this core only a few percent (up to 10 %) of chevrons are
preserved, with the rest made up of clear halite filling dissolution
pipes. In earlier work on ancient bedded halite (e.g. Wardlaw and
Schwerdtner, 1966; Wardlaw and Watson, 1966), it was argued that it was
impossible to produce such mature chevron halite layers by
synsedimentary dissolution-reprecipitation alone. The main argument
against the dissolution-reprecipitation process was the assumption that
the entire salt layer would collapse if it were pervasively truncated
by dissolution pipes. These authors thus considered recrystallization
as a likely additional effect for producing those features. However,
others (Shearman, 1970; Lowenstein and Hardie, 1985; Casas and
Lowenstein, 1989) pointed out that the structure can entirely be
explained by a repeated dissolution-reprecipitation processes, and
argued that recrystallization is not necessarily required for producing
that feature. In this study, the lack of evidence for extensive
deformation-induced recrystallization processes (see below) in the
truncated chevron layer supports the view that deformation-induced
recrystallization was not involved in development of those structures.

5.2
Deformation-related structuresThe majority of the grains in
the samples studied contain subgrains (Figs. 7 and 9). The polygonal
shape of the subgrains suggests deformation dominated by
climb-controlled creep (Senseny et al., 1992), although some
microstructural evidence may point to the contribution of cross-slip is
also present (Fig. 10). The lack of evidence for flattened grains
points to a total strain of <10 % (Jackson, 1985). Geological maps
of the Twenthe-Rijn area (Geluk and Duin, 1997; Doornenbal et al.,
2002) show that the salt is displaced along NW-SE trending faults, and
the most plausible assumption is to associate the deformation of the
salt layers with these faults and thus to the Cretaceous inversion
tectonic event (de Jager, 2003).Subgrain-free regions,
elongated subgrains at grain boundaries and bulged grain boundaries
provide clear evidence for strain energy driven grain boundary
migration (Drury and Urai, 1990; Bestmann et al., 2005). Observations
on both etched and irradiated samples show that new, less substructured
halite is replacing highly substructured grains (Figs. 9, 11-15). In
many cases the migrating grain boundaries consume old, milky,
fluid-inclusion-rich parts of a grain (Figs. 12 and 16). Since the area
that was swept by grain boundary migration is fluid-inclusion-free, it
seems very likely that this process transfers brine into the grain
boundaries and provides conditions for pressure solution creep (Urai,
1983; Urai et al., 1986; Spiers et al., 1990). Although grain boundary
migration is common, the recrystallized volume is only a few volume
percent (Fig. 9). Sizes of entirely
substructure-free grains vary between the size of a subgrain and a few
millimeters (Figs. 9 and 11), suggesting that formation of new
high-angle grain boundaries by progressive subgrain rotation is also
possible in salt in nature (Drury and Urai, 1990). To date, subgrain
rotation has only been documented in halite experimentally deformed dry
and at high temperature (Guillopé and Poirier, 1979; Franssen,
1993). Although microstructural evidence reported here points to the
presence of subgrain rotation recrystallization, an EBSD analysis of
selected regions (e.g. Fig. 11) is necessary and in progress to address
this question in more detail.

Water content in the Röt
salt is high owing to the numerous primary fluid inclusions (e.g. Fig.
9) and, as shown above, evidence for grain boundary migration is
present. It would be expected that as a grain boundary sweeps through a
milky, fluid-inclusion-rich area and collects the fluid inclusions,
this process amplifies (since fluid increases grain boundary mobility)
and continues until the majority of the grains recrystallize and the
driving force is eliminated. Considering that the deformation (and
creation of driving force) was at least 65 Ma years ago, it is puzzling
to see that so many of the deformed grains are not recrystallized,
because halite recrystallizes readily at room temperature (Schenk and
Urai, 2004). One hypothesis for this phenomenon could be the change in
the burial depth, and thus a change in temperature. According to the
burial history of Röt strata in vicinity of Hengelo (NITG, 1998),
the salt strata were deformed at a maximum burial depth of 1.2 to 1.5
km (Jurassic and Cretaceous). Keeping in mind the recent depth of the
beds (<500 m), however, it is unlikely that this drop in temperature
(about 30 ºC) had a major effect on the rate of recrystallization.
Another explanation could be a reduction in driving force (recovery,
subgrain formation) or a reduction in grain boundary mobility due to a
change in fluid structure at grain boundaries (Urai et al., 1986; Peach
et al., 2001; Schenk and Urai, 2004).

5.3
Differential stress and strain rate calculations In most materials which
deform by dislocation creep processes, the steady state subgrain size
is inversely proportional to the stress difference (σ1
- σ3) and
independent of other variables. The relation is:

(1) D = k σ -m

where D is the average
subgrain size in µm, k and m are material constants and σ is the
differential stress in MPa. Two recent datasets exist for
experimentally deformed rock salt (Carter et al., 1993; Franssen, 1993)
where subgrain size and stress were measured. Plotting those data
together, the parameters of the least squares fit line become k = 215
and m = -1.15 (correlation coefficient = 0.90). In this paper we used
these values for the stress calculation. For the subgrain diameter (D)
calculation, subgrain boundaries from reflected light photographs of
etched thin sections and transmitted light photographs of irradiated
thin sections were digitized and then analyzed with the NIH Image
software (http://rsb.info.nih.gov/nih-image/index.html). The software
calculates the area of every subgrain, and the D value is obtained by
calculating the diameter of a circle equivalent in area to that of the
subgrain. The calculated differential
stress values (Table 1) are in good agreement with those reported for
various bedded rock salts by Carter et al. (1993), and imply that the
Hengelo salt underwent deformation at stresses of 0.5 to 1 MPa.

Theoretically, based on the
distribution of differential stress magnitudes over the studied salt
layers, it would be possible to characterize the type of deformation
(simple shear vs. pure shear). This is because in case of simple shear
one would expect the same differential stress values over the whole
salt succession, while in case of pure shear the highest differential
stress values would be expected at the top and at the bottom, the
lowest values at the middle of the deforming salt succession. Although the calculated mean
differential stress values are slightly different in the four samples
studied, the differences are not significant due to the overlapping
range of predicted values (Figs. 18 and 19), preventing us from
characterizing the type of the deformation (e.g. simple shear vs. pure
shear).

where ε is the total strain
rate in s-1, εCS
and εCLare
strain rates from dislocation creep processes, and εPS
is strain rate
from pressure solution (by definition zero for dry salt).
Microstructures observed in this study are consistent with
climb-controlled and the fluid assisted diffusional-creep (c.f. Fig. 1
of Schenk and Urai, 2004). Thus it seems reasonable to use the
corresponding constitutive equations for climb controlled creep
(equation 3) and for pressure solution creep (equation 4) to calculate
total strain rate.

where strain rate (ε) is
expressed in s-1, the pre-exponential
constant is in MPa-ns-1,
apparent
activation energy is in Jmol-1, Boltzmann’s
gas constant (R) is in
Jmol-1K-1,
temperature (T) is in ºK, differential stress (σ1
- σ3)
is in MPa and grain size (D) is in mm. For the calculations we propose
that the deformation occurred at a depth of ~1.5 km in the Cretaceous
time. Since we are lacking of paleogeothermal data for the study area,
we used that of present day (~30-35 ºC/km; Haenel, 1980), and thus
T=323 ºK.Calculated strain rate values
(Table 2) show that in finer grained samples, solution precipitation
creep contributes to total strain rate at least in the same order of
magnitude as dislocation creep processes. In the relatively
coarse-grained sample (C layer, 430.9 m) dislocation creep is the main
deformation mechanism.

Table 2.
Strain rate
calculated from equation (3) and equation (4). Total strain rate ( ε )
is the sum of the two values (equation 2).

Sample

No.
of analyzed grains

Mean grain diameter
(mm)

1 S.D.

εCL (s-1)

εPS (s-1)

ε (s-1)

“D”
layer (423.3
m)

24

6.01

2.08

6.31x10-14
– 3.33x10-13

4.31x10-13
– 7.03x10-13

4.94x10-13
– 1.04x10-12

“C”
layer (430.9
m)

3

25.91

12.59

4.20x10-14
– 2.40x10-13

4.77x10-15
– 7.96x10-15

4.68x10-14
– 2.47x10-13

“C” layer (443.5
m)

15

8.80

5.28

2.34x10-14
– 1.49x10-13

1.02x10-13
– 1.76x10-13

1.26x10-13
– 3.25x10-13

“A” layer (453.5
m)

52

4.91

2.82

3.51x10-14
– 2.07x10-13

6.25x10-13
– 1.12x10-12

6.97x10-13
– 1.32x10-12

Fig. 18.
Logarithmic subgrain
size vs. logarithmic differential stress of experimental data from
Carter et al. (1993) and Franssen (1993). The least squares fit line of
the combined data set was used to calculate the differential stress for
the Hengelo samples (Table 1).

Fig. 19.
Graph shows the
calculated differential stress and the strain rate distribution over
the inspected salt layers. For discussion see text.

Experiments on bedded and
domal salts (Wawersik and Zeuch, 1986; Hunsche et al., 2003) showed
that while the form of the constitutive laws governing creep rate is
similar, actual creep rates in different types of salts vary over a
factor of ±10. This variation in creep rate can be attributed to
secondary phases, solid solution impurities, grain size and dislocation
density (Pfeifle et al., 1995). In our calculations we took a factor of
10 to be a plausible range. The uncertainty in the creep rate is too
large to allow detection of significant differences in strain rate
across the layers (Fig. 19).

6.
ConclusionsIn this paper, with the aid
of microstructural analysis, we differentiated between primary
(synsedimentary) and secondary (deformation-related) microstructures
present in the Röt salt. Most of the primary structures are still
preserved and modifications by recrystallization have been subordinate.
In our samples, recrystallization of salt is incipient, but it shows
how associated processes erase primary structures, making
identification of synsedimentary features progressively more difficult
as deformation and recrystallization continues. Nevertheless, in the
case of slightly deformed salts, there are already microstructures,
e.g. grain boundary migration-type microstructures (c.f. Fig. 15 and
16), which, without a careful microstructural analysis, could be
misinterpreted. One other important implication of this paper is the
evidence for a process (grain boundary migration) that transports and
concentrates primary fluid-inclusion brines into grain boundaries. This
implies that the amount of brine at the grain boundaries increases as
the grain boundaries sweep through a fluid-inclusion-rich area, thus
giving rise to pressure solution creep, which accordingly can
contribute to the total strain rate at least in the same order of
magnitude as dislocation creep processes. Keeping in mind that many of
the bedded halites are of salt pan origin and thus rich in fluid
inclusions, it is likely that pressure solution creep is significant in
halite alongside dislocation processes, at least in the early phases of
halokinesis.

AcknowledgementsThe authors thank Wim Paar
(Akzo Nobel company) for providing the salt core samples, Manfred
Thomé at the Jülich Forschungszentrum for carrying out the
gamma-irradiation and H. W. den Hartog (University of Groningen) for
helpful discussion on gamma-irradiation. This work was performed as a
part of the SPP 1135 project (nr. UR 64/5-1-2), and was financed by the
DFG (Deutsche Forschungsgemeinschaft). The manuscript benefited from
thorough reviews by Timothy Diggs and Chris Spiers.